It has been clarified in the art that an insulation deterioration of a power cable such as a rubber or plastic insulated power cable is mainly caused by the absorption of moisture into the solid insulation material to which a rated line voltage is applied. In the circumstance, water tree is induced in the insulation material, and is then developed therein to finally result in a breakdown of the power cable. Therefore, a method for detecting the insulation deterioration including water tree is required to be established, thereby avoiding the breakdown of an insulation material for a power cable in advance.
One type of a conventional method for diagnosing an insulation deterioration of a power cable comprises steps of applying a DC high voltage to an insulation of the power cable, and detecting leakage direct current flowing through the insulation, so that the insulation deterioration is diagnosed dependent on a level of the leakage direct current. The other type of a conventional method for diagnosing an insulation deterioration of a power cable comprises steps of applying a DC voltage to the power cable, and analyzing motion and state of electrons in an insulation thereof to detect the insulation deterioration of the power cable.
In the conventional methods, however, there is a disadvantage that these methods are not applied to a power cable to which a rated load voltage is applied to supply electric power to actual loads, because the aforementioned DC voltage which is determined in diagnosis conditions is applied to the power cable. Even worse, there is a further disadvantage that the insulation deterioration is badly promoted to result in the breakdown of the power cable in some case dependent on a degree of the insulation deterioration, because the diagnosing DC high voltage is applied to the power cable.
In view of these situations, the inventors have conducted research and development for a diagnosis of an insulation deterioration intensively, and found that a direct current component is included in charging current flowing through an insulation of a cross linked polyethylene insulated poly-vinylchloride sheathed cable (simply defined "CV cable" hereinafter) to which an AC voltage is applied in a case where the insulation is deteriorated by water tree. Further, the inventors have concluded that an insulation deterioration of a CV cable could be detected in accordance with a magnitude and a polarity of the direct current component under a state that the CV cable has supplied electric power to actual loads connected thereto. In this diagnosis of an insulation deterioration, a grounding conductor of the CV cable is utilized to detect charging current. Therefore, the diagnosis method is a very safe method for operators.
In a case where the insulation deterioration is diagnosed in accordance with the magnitude and the polarity of the direct current component as described above, however, experienced knowledge and much labor are required to obtain precise diagnosis results. This is because direct current component measured in a CV cable in an actually installed site ranges nA to .mu.A widely, and it must be decided to what extent the direct current component is affected by stray earth current, especailly, where the direct current component is in the range of nA. A minute level of the stray current flows from the ground through a corrosion-proof layer (generally called "sheath") of the CV cable thereto where an insulation resistance value of the corrosion-proof layer is decreased down to a small value, although the insulation resistance value is more than 2,000 M.OMEGA. at the time of manufacture and installation thereof. For this reason, it is necessary to measure an insulation resistance of the corrosion-proof layer and to detect the presence or non-presence of a power source providing direct current potential to be applied to the corrosion-proof layer. This is a reason why much labor is needed as described above.
In a case where it is decided that a measured current value is affected by the stray current, the direct current component from the CV cable and the stray current from the ground must be separated from each other as precisely as possible, so that the insulation deterioration is diagnosed based on the separated direct current component.
For this purpose, a method for diagnosing an insulation deterioration of a power cable in which a direct current component is precisely detected as deterioration signal produced in a power cable having water tree in an insulation thereof has been proposed in Japanese Pat. application No. 61-286972 filed on Dec. 2, 1986 (Japanese Pat. Laid-open No. 63-139261 laid-open on June 11, 1988). The method for diagnosing an insulation deterioration of a power cable comprises steps of measuring first positive and negative direct current components by applying positive and negative DC voltages across a metal shielding layer of a rubber or plastic insulated power cable and the ground in the presence of an electric resistance therebetween respectively, measuring a second direct current component without the resistance by applying no DC voltage across the metal shielding layer and the ground, and separating direct current component and stray current included in charging current of the rubber or plastic insulated power cable in calculation based on the first positive and negative direct current components and the second direct current component, so that the insulation deterioration of the rubber or plastic insulated power cable is diagnosed in accordance with the separated direct current component.
According to the method for diagnosing an insulation deterioration of a power cable as proposed in the Japanese Pat. application No. 61-286972, however, there are disadvantages that operation in which the three direct current components are measured is troublesome, although the precise measurement thereof can be realized, and that there is limitation in decreasing a calculation amount, because the direct current component and the stray current are separated in the calculation based on the three direct current components.